Coating composition, a method for coating a substrate, a coated substrate, a packaging material and a liquid package

ABSTRACT

The present invention relates to a curtain coatable gas barrier coating composition comprising a polymer and a surfactant, wherein the polymer is selected from a group consisting of polyvinyl alcohol and polysaccharides, or mixtures thereof, wherein said polysaccharides are soluble or dispersable or suspendable in water and the surfactant is a water soluble non-ionic ethoxylated alcohol. The present invention also relates to a method for providing a substrate with a gas barrier layer by means of the coating composition, and a coated substrate having at least one gas barrier layer obtained by coating the substrate with the coating composition. Further, the invention relates to a packaging material comprising a coated paperboard coated with the coating composition, and a liquid package comprising such a packaging material.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a coating composition comprising apolymer and a surfactant, wherein the polymer is selected from a groupconsisting of polyvinyl alcohol and polysaccharides, or mixturesthereof. In particular it relates to a gas barrier coating compositionsuitable for curtain coating. The present invention also relates to amethod for providing a substrate with a gas barrier layer by means ofthe coating composition, a coated substrate, a packaging materialcomprising a coated paperboard, and a liquid package comprising such apackaging material.

BACKGROUND OF THE INVENTION

Curtain coating is a coating process in which a linear flow of a liquidcoating composition is deposited on a surface of a moving substrate,such as paper web. The coating composition forms a liquid sheet thatfalls freely before impinging onto the moving substrate that is to becoated.

Curtain coating is a technique well suited for coating a paper web witha liquid barrier layer, since the barrier substance must form a filmthat covers the surface entirely, or as close to entirely as possible.However, the curtain coating process is susceptible to certain defects,such as irregular heel formation, air entrainment and curtaininstabilities that may result in a coated surface that displays smalleror bigger areas that lacks coating. In order to discover such defects inthe coverage of the coating, which can be very small due to e.g. airbubbles in the liquid coating, a test may be made by applying a colouredtest liquid onto the coated surface. Any defects will then appear ascoloured areas or so called pinholes. For some applications it isparticularly important that the coating does not contain any defects orat least very few defects, for example in the food industry and forliquid packages (i.e. packages containing liquids).

It is previously known that polysaccharides and polyvinyl alcohol aregood barriers against gases. A coating comprising polysaccharides and/orpolyvinyl alcohol deposited on a substrate is known to be suitable as abarrier against oxygen, odours, aromas, etc. in for example packagingmaterial based on paperboard substrate.

In curtain coating a suitable dynamic surface tension, calculated fromMach-angle measurements in the falling curtain, is required in order forthe free falling curtain of coating liquid to form a stable curtain andnot to have defects in the form of holes. Typically, for Newtonianfluids, the coating liquid should have a dynamic surface tension of lessthan 40 mNm to satisfy the criteria of a falling curtain without theformation of holes. However, polysaccharides and polyvinyl alcohol,which are non-Newtonian fluids does have a considerably higher surfacetension which leads to an instable curtain at low flow rates.

In order to reduce the surface tension of polyvinyl alcohol it has beenproposed to add a surface active substance, a surfactant, to thepolyvinyl alcohol, for example as described in EP 2182113, using asurfactant of the type Air Products EnviroGem AE03. However, it has beenfound that even though the stability of the curtain improved when usinga surfactant, the barrier effect of the coating deteriorated, and tosuch a degree that it was not usable for producing liquid packages. Asother examples, it has been proposed to add relatively large amounts ofethanol or similar alcohol to the polyvinyl alcohol, but this is howevernot a satisfying solution of the coating problem in the paper industrysince it requires handling and recycling of resulting volatile organiccompounds (VOC).

To summarise, in order to use curtain coating to obtain a barrier on asubstrate, such as a polyvinyl alcohol or polysaccharide barrier on afibre substrate, e.g. paperboard, it is necessary to use addedsubstances that reduces the dynamic surface tension in order to have astable process, but these substances will at the same time have anegative effect on the film formation and therefore the barrier will bedefective.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved coatingcomposition suitable for curtain coating, in particular for the abovementioned fields of application.

Another object is to obtain a coating that will result in a finalproduct that is suitable for the food industry, that is free of anytoxic or unhealthy substances, and which is safe to use. Moreover, whenused in a packing material, it must not contain any substances that canmigrate into the food product or emit any smells or aromas when used infood or liquid packages.

The objects are achieved by the present invention.

According to a first aspect of the present invention is defined acurtain coatable gas barrier coating composition comprising a polymerand a surfactant, wherein the polymer is selected from a groupconsisting of polyvinyl alcohol and polysaccharides, or mixturesthereof, wherein said polysaccharides are soluble or dispersable orsuspendable in water and the surfactant is a water soluble non-ionicethoxylated alcohol. This coating composition makes it possible toobtain a stable curtain with a low number of pinholes in the coating ofa coated substrate, such as a fibre substrate, e.g. a paperboard. Thiscoating provides good barrier qualities against gases, e.g. oxygen,aromas and odours.

Preferably, the polysaccharides may comprise starch, starch derivatives,MFC (micro fibrillar cellulose) and hemicellulose.

According to one embodiment of the present invention, the polymercomprises polyvinyl alcohol alone or in a mixture with starch and/or astarch derivative and/or MFC and/or hemicellulose.

According to another embodiment, the polymer comprises starch alone, ora starch derivative alone, or a mixture thereof, or either one inmixture with MFC and/or hemicellulose.

According to yet another embodiment, the polymer comprises MFC alone orhemicellulose alone, or a mixture thereof.

According to a further embodiment of the present invention, thesurfactant may be a saturated iso-C13 alcohol. According to a furtherfeature of this embodiment, the surfactant has the formulaRO(CH₂CH₂O)_(x)H, wherein R═iso-C₁₃H27 and x is 8 or higher

According to another embodiment of the present invention, the surfactantmay be an alkyl polyethylene glycol ether based on C10-Guerbet alcohol,with a degree of ethoxylation of 7 or higher.

According to yet another embodiment of the present invention, thesurfactant may be a saturated short-chain fatty alcohol. According to afurther feature of this embodiment, the surfactant has the formulaRO(CH₂CH₂O)_(x)H, wherein R═a saturated short-chain alcohol and x is 5or higher.

The coating composition may comprise a concentration of 0.05-2.0 weight% of non-ionic ethoxylated alcohol surfactant, preferably 0.1-1.0%, andmore preferably 0.2-0.5 weight %, of the total weight of the bone drycoating composition.

The solid content of the composition may be 4-20%; preferably 7-15%, or6-12%, or even 8-10%. The balance is water.

Further, the coating composition may comprise platy particles, forexample it may comprise Montmorillonite clay in nano particle form. Thishas the advantage of improving the gas barrier function. The clay orother suitable platy or nano form additives may be added to thepolyvinyl alcohol in an amount of 1-20% dry weight of the total dryweight of the composition, preferably 3-15% and more preferably 5-10%.

The coating composition may preferably comprise a weight percentage ofapproximately 80-99.5% of dry matter in the composition constituted by apolymer chosen as the main component, and the amount of additives to themain component, such as other polymers or platy particles andsurfactant, have a weight percentage of approximately 0.05-20% of drymatter in the composition.

Preferably, the weight percentage of the polymer chosen as the maincomponent is 85-98%, and the weight percentage of the additives is2-15%. However, it should be emphasized that any combination of thementioned substances, and in any weight proportions, is conceivablewithin the frame of the invention.

According to a second aspect of the invention is defined a method forproviding a substrate with a gas barrier layer in at least one coatingstep, comprising application of a coating composition as defined in anyone of the claims defining the coating composition, by curtain coatingthe substrate with the coating composition in at least one coating step.

Accordingly, the method may comprise only one coating step of applying acoating layer of the described composition of polyvinyl alcohol alone,or of any of the mentioned polysaccharides (starch, starch derivative,MFC, hemicelluloses) alone, or of mixtures thereof wherein polyvinylalcohol is the main component or starch or starch derivative or MFC orhemicellulose is the main component. Any combination is foreseeable.

The method may further comprise a second step of applying a secondcoating layer with a composition chosen from all the above mentionedpossibilities. Usually there will be a drying step after each coatingstep. The coating composition chosen for the first and second layer maybe the same coating composition or different coating compositions.

Accordingly, the method may comprise curtain coating the substrate in afirst step with a coating composition containing polyvinyl alcohol andin a second step with a coating composition containing polyvinylalcohol.

Alternatively, the method may comprise curtain coating the substrate ina first step with a coating composition comprising a polysaccharide andin a second step with a coating composition comprising a polysaccharide.

In another alternative, the method may comprise curtain coating thesubstrate in a first step with a coating composition containing apolysaccharide and in a second step with a coating compositioncomprising polyvinyl alcohol.

In yet another alternative, the method may comprise curtain coating thesubstrate in a first step with a coating composition containingpolyvinyl alcohol and in a second step with a coating compositioncomprising a polysaccharide.

In still a further alternative, the method may comprise curtain coatingthe substrate in a first step with a coating composition comprisingpolyvinyl alcohol, MFC, clay in nano particle form and a water solublenon-ionic ethoxylated alcohol, and curtain coating the substrate in asecond step with a coating composition comprising starch or a starchderivative and a water soluble non-ionic ethoxylated alcohol. Theopposite order of the coatings is also possible.

In another alternative, the method may comprise curtain coating thesubstrate in a first step with a coating composition comprisingpolyvinyl alcohol, hemicellulose, clay in nano particle form and a watersoluble non-ionic ethoxylated alcohol, and curtain coating the substratein a second step with a coating composition comprising starch or astarch derivative and a water soluble non-ionic ethoxylated alcohol orvice versa.

Additional coating steps are of course also foreseeable, and anycombination of layers of the same or different coating compositions,from the whole range of possible coating compositions, is possible. Anexample of an additional coating step is a pigment coating step, whichmay very well be made before coating with the gas barrier coatingcomposition.

Further, the coat weight of the at least one coating layer may be 0.4g/m² or more and less than 7.0 g/m². The coat weight of the barrierlayer may be 0.8 g/m² or more and less than 7.0 g/m², and preferably1.2-4.0 g/m², and even more preferably 1.6-3.2 g/m².

According to one feature, the substrate may be a fibre substrate or apaper or a paperboard. However, it is conceivable to implement theinvention on other substrates, without going beyond the scope of theinvention.

According to a third aspect of the present invention is defined a coatedsubstrate having at least one gas barrier layer obtained by coating asubstrate with a coating composition as defined in any one of claimsdefining the coating composition.

The gas barrier layer may be a barrier against oxygen in particular, andgases including odours and/or aromas.

Further, the coated substrate may have a gas barrier layer that has acoat weight of 0.8 or more and less than 7.0 g/m² and preferably 1.2-4.0g/m², and even more preferably 1.6-3.2 g/m².

In one embodiment, the coated substrate may be a paperboard comprisingat least one fibre based layer coated with the coating composition.Alternatively, the substrate may be a paper or any other substraterequiring the concerned type of barrier.

According to a fourth aspect of the present invention, is defined apackaging material comprising a coated paperboard as defined above,further comprising a layer of low density polyethylene on each side ofthe coated paperboard, and having an OTR value of less than 40 ml/m²·24h·1 atm, or preferably even less than 10 ml/m²·24 h·1 atm. OTR stand foroxygen transmission rate and the instrument used for the measurement isMocon Ox Tran 2/21.

According to a fifth aspect of the present invention is defined a liquidpackage made from a packaging material as defined above.

As mentioned before, typically, for Newtonian fluids, the coating liquidshould have a dynamic surface tension of less than 40 mNm to satisfy thecriteria of a falling curtain without the formation of holes. However,according to the present invention it has been advantageously found thatpolysaccharides and polyvinyl alcohol, which are non-Newtonian fluids,do not necessary need as low surface tension as Newtonian fluids due totheir elasticity. There is then a need to obtain a good relationshipbetween the flow rate and the surface tension and the non-newtonianfluid characteristics. Some common concepts of curtain stability are theminimum flow which can be reached without rupture of the liquid curtainand the minimum flow at which the curtain heals itself to a completelydefect free liquid curtain. In accordance with the present invention, ithas surprisingly been found that the surface tension for polysaccharidesand polyvinyl alcohol may have dynamic surface tension values of up to50 mNm, and still have a defect free operation even at very low flowrates such as lower than 4.0 litres/minute*meter.

Further features and advantages will become apparent from the followingdetailed description of the invention and test examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, with reference beingmade to test examples and the enclosed schematic drawings illustratingdifferent aspects and embodiments of the invention, given as examplesonly, and in which:

FIGS. 1 a-1 d shows schematically examples of a coated substrateaccording to the present invention, and

FIGS. 2 a-2 d shows schematically examples of a packaging materialaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

During the search for a suitable surfactant and a suitable method, manydifferent commercially available surfactants were tested that werealleged to be suitable for curtain coating. However, they were all foundunsuitable since they all failed either in providing a stable curtain orin providing a sufficiently good barrier.

After having performed many tests it was surprisingly found a group ofsurfactants that managed to provide the desired result of a stablecurtain and a high quality barrier with an acceptable low amount ofpinholes. This group of surfactants was non-ionic ethoxylated alcohols,such as sold under the trade names of Lutensol ON and Lutensol TO, byBASF. In particular Lutensol ON50, Lutensol ON70 and Lutensol TO8 werefound to provide the required results.

According to the product information supplied by BASF, the Lutensol ONtypes are non-ionic surfactants. They are alkyl polyethylene glycolethers made from a saturated synthetic, short-chain fatty alcohol. Theyconform to the following formula: RO(CH2CH2O)xH, where

R=saturated, synthetic, short-chain fatty alcohol

x=3, 5, 6, 7, 8 or 11

The numeric code in the product name usually indicates the degree ofethoxylation. The Lutensol ON types are manufactured by causing thefatty alcohol to react with ethylene oxide in stoichiometricproportions. The ethoxylation temperature is kept as low as possible.This, combined with the high purity of the feedstocks, ensures thathigh-performance products with low toxicity are obtained. According toadditional information available from BASF, R may be iso-C10.

According to the product information supplied by BASF, the Lutensol TOtypes are non-ionic surfactants. They are made from a saturated iso-C13alcohol. They conform to the following structural formula:RO(CH2CH2O)xH, where

R=iso-C13 H27

x=3, 5, 6, 6.5, 7, 8, 10, 12, 15 or 20

The numeric code in the product name indicates the degree ofethoxylation. The Lutensol TO types are manufactured by causing theiso-C13 oxo alcohol to react with ethylene oxide in stoichiometricproportions. The ethoxylation temperature is kept as low as possible.This, combined with the high purity of the feedstocks, ensures thathigh-performance products with low toxicity are obtained.

In the case of Lutensol ON50, x (the degree of ethoxylation) is 5, andin the case of Lutensol ON70, x is 7. A Lutensol ON having a lowermolecular weight, Lutensol ON30 (x=3), was also tested but did notresult in the desired effects.

In the case of Lutensol TO8, x is 8. Lutensol TO8 has a molecular weightof approximately 600 g/mol. Also for this group, a Lutensol TO having alower molecular weight, Lutensol TO6 (x=6), was also tested but did notresult in the desired effects.

The non-ionic ethoxylated alcohols having the desired effect are watersoluble and provide relatively slow wetting. Lutensol ON30 and TO6, onthe other hand, are not water soluble. Lutensol ON is water soluble fora degree of ethoxylation of 5 and above, while Lutensol TO is watersoluble for a degree of ethoxylation of 8 and above.

Another such water soluble non-ionic ethoxylated alcohol is Lutensol XP.According to the product information supplied by BASF, the Lutensol XPtypes are branched non-ionic surfactants. They are alkyl polyethyleneglycol ethers based on C₁₀-Guerbet alcohol and ethylene oxide. TheLutensol® XP types are manufactured by reacting the C₁₀-alcohol withethylene oxide in stoichiometric proportions. The numeric portion of theproduct name indicates the general degree of ethoxylation. The productis water soluble when the degree of ethoxylation is 7 or higher.

Tests with good and satisfying results were performed with a liquidcoating composition comprising polyvinyl alcohol, of which a solutionwas prepared according to the instructions on the particular product,and to which solution the concerned surfactant was added according tothe following:

-   Concentration of surfactant (weight % of added surfactant as from    package of the weight of the total bone dry composition): 0.05-2%;    preferably 0.1-1.0% or even 0.2-0.5%; Solid content of the    composition: 4-20%; preferably 7-15%, or even 8-10%.

The tests were made by depositing the liquid coating composition on afibre based moving web in a curtain coating process. The web waspaperboard.

Test results are listed in Table 1. The results in the table aremeasured on paperboard coated in a curtain coating process with twolayers of the coating composition according to the invention, each layerbeing equal or not in coat weight to the other layer, totalling 2.0-5.0g/m². In this table are also shown comparative tests performed with acoating comprising polyvinyl alcohol (PvOH) alone and a coatingcomposition comprising polyvinyl alcohol and the by Cytec, USA,commercially available surfactant Aerosol OT75 comprising “Sodiumdioctyl sulfosuccinate”, which did not result in a satisfying gasbarrier.

TABLE 1 Test examples Coat AO2IR OTR weight Visual Curtain KIT-Pinholes/ ml/dm2* ml/m2*24 Coating composition g/m2 observationstability number dm2 24 h h*1atm PvOH 3.2 OK at high Holes in 1 237 18175.8 flow rate the film only PvOH + 0.2% Aerosol OT75 2.4 OK OK 1 2525152 61.8  PvOH + 0.5% Lutensol 3.2 OK OK 3 108 556 4.9 ON50 PvOH + 10%Cloisite + 0.5% 3.2 OK OK 3 99 397 2.6 Lutensol ON50 PvOH + 0.3%Lutensol 3.0 OK OK — 4 51 7   ON70 PvOH + 10% Cloisite + 0.3% 2.0 OK OK12 4 94 5.2 Lutensol ON70 PvOH + 10% Cloisite + 0.3% 3.0 OK OK 12 3 132.9 Lutensol ON70 PvOH + 10% Cloisite + 5% 3.2 OK OK 5 84 511 6.2 MFC +0.3% Lutensol ON70 First layer 3.2 OK OK 7 39 552 8.1 (PvOH + 10%Cloisite + 5% (1.6 + 1.6) MFC + 0.3% Lutensol ON70) + Second layer(Starch + 0.2% Lutensol ON70) Starch + 0.2% Lutensol 5.0 OK OK 5 327 88131.0  ON70 KIT = TAPPI T 559 KIT test (repellency of paper and board togrease, oil, and waxes) AO2IR is short for Ambient Oxygen Ingress Ratewhich is a method for measuring oxygen gas transmission with a Perm-Mateinstrument supplied by PBI Dansensor or Systech Illinois. OTR is shortfor Oxygen Transmission Rate, measured at 23° C. and 50% RH by theinstrument Mocon Ox Tran 2/21. A low KIT number indicates resistanceonly against fat; a high KIT number indicates high resistance againstfat and solvents. A low AO2IR value indicates high resistance againstgas transmission. The OTR is measured on a packaging material comprisingthe inventive coated paperboard provided with a polyethylene layer of 30g/m2. A low OTR value indicates good barrier function.

The coating composition may also comprise Montmorillonite clay, asmentioned before, or talc, kaolin clay or other similar solid additivesin platy form or nano-particle form, while maintaining good barrierresults, which has also been proven in tests. The tests performed withMontmorillonite (Cloisite) are also shown in Table 1. The clay or othersuitable platy or nano form additives may be added to the polyvinylalcohol in an amount of 1-20% dry weight of the total dry weight of thecomposition, preferably 3-15% and more preferably 5-10%.

Particularly good barrier results are the results showing a value of theoxygen transmission rate (OTR) of less than 40 ml/m2*24 h*1 atm, or evenless than 10 ml/m2*24 h*1 atm. The same tests were also made in whichpolyvinyl alcohol was exchanged for starch. Also the liquid coatingcomposition comprising starch and surfactant showed good test resultswith an OTR of less than 40 ml/m2*24 h*1 atm, as can be seen in thetable.

In addition, the same tests were performed in which a coating had beenobtained by having a first layer with a liquid composition comprisingpolyvinyl alcohol, Montmorillonite clay and MFC together with asurfactant, and a second layer with a liquid composition comprisingstarch and a surfactant. This coating also resulted in good andsatisfying test results, as shown in table 1.

It can be foreseen that, based on the good test results with starch andMFC, also hemicelluloses, primarily Xylanes and Xyloglucanes, which arealso known to be good barriers alone or in mixture, can be used in thesame way as starch and MFC.

The tests were performed with a velocity of the web substrate between420 and 850 m/min with good results. With a curtain flow rate of atleast 6 l/min*m and stable curtain conditions, it can be shown that itis theoretically possible to have a velocity of 250-1500 m/min, or evenmore, with a coat weight of 0.4-4.0 g/m² applied in two layers,totalling 0.8-8.0 g/m², and achieve satisfactory barrier results.

It should be generally understood that the present invention is notlimited to the surfactant products with the above trade names, but theremay be similar/corresponding products sold under other trade names byother suppliers or manufacturers which will work accordingly. It shouldalso be mentioned that a possibility is foreseen to mix two or moresurfactants, and it can also be foreseen that other components may beadded to the coating composition, without departing from the scope ofthe present invention.

The polyvinyl alcohols used for the tests were supplied from KurarayNordic Oy in the form of product range Mowiol 6/98, 15/99 and 28/99, andit was prepared by dissolving in hot water according to the supplier'sinstructions. The starch used was a hydroxipropylated oxidized starchproduct Solcoat P150 supplied by Solam AB. The starch was prepared bydissolving in hot water according to the suppliers instructions. Themicrofibrillar cellulose (MFC) was supplied by Innventia AB and wassupplied as a fibre suspension ready to use. The Montmorillonite claywas supplied by Southern Clay Products, USA in the form of the productCloisite Na+. Tests were also made with kaolin clay and talc, withsimilar good results as with Montmorillonite. The kaolin clay used wassupplied by Imerys in the form of Product Barrisurf LX. The talc usedwas supplied by FinnTalc Oy in the form of the product Finntalc C10B. Itshould be clear that also other similar types of polyvinyl alcohols, andother brands of the mentioned substances may be used without departingfrom the scope of the present invention. It is concluded that thepresent invention makes it possible to obtain a barrier coatedpaperboard that is suitable for liquid packages, in which the polyvinylalcohol/polysaccharide layer provides an oxygen barrier and otheradditional layers, e.g. polyethylene, provides a liquid barrier andsealing properties.

The concerned liquid coating composition can generally be prepared byadding the used polymer or polymers (polyvinyl alcohol and/orpolysaccharides) and also Montmorillonite (or talc or kaolin clay) whenapplicable, in powder form to cold water. If MFC or hemicellulose isused it is generally in pre-suspended form. Thereby a liquid is obtainedin which the added substances are dissolved or suspended or dispersed inwater. The liquid is then heated. After preparation, the liquid shall beallowed to cool down before the surfactant is added and then theresulting composition is ready to be used in the curtain coatingprocess. Alternatively, each of the substances may prepared by itself,e.g. be dissolved (or suspended or dispersed in cold water) beforemixing the substances with each other, heating and then adding thesurfactant in order to obtain the final composition.

In FIGS. 1 a-1 d is schematically illustrated examples of a coatedsubstrate according to the present invention. In FIG. 1 a is shown acoated substrate where the substrate 1 is a paperboard comprising atleast one fibre based layer, and the paperboard is coated with a gasbarrier coating layer 2 comprising a coating composition according tothe present invention. In FIG. 1 b, the coated substrate is furtherprovided with a pigment coating layer 3 on the opposite side of thepaperboard layer 1, i.e. on that side of the paperboard layer 1 that isnot coated with the barrier coating 2. In FIG. 1 c, the coated substrateof FIG. 1 a is also provided with a pigment coating layer 3, but in thisexample the pigment coating layer 3 is provided between the paperboardlayer 1 and the barrier coating layer 2. Consequently, the barriercoating 2 has been applied on top of the already applied pigment coatinglayer 3. Finally, in FIG. 1 d, the coated paperboard of FIG. 1 c hasbeen provided with an additional pigment coating layer 3, resulting in apaperboard layer 1 having a pigment coating layer 3 on both sides and abarrier coating layer 2 applied on one of the pigment coating layers 3.

FIGS. 2 a-2 d shows schematically examples of a packaging materialaccording to the present invention. FIGS. 2 a-2 d illustrates a coatedsubstrate corresponding to FIGS. 1 a-1 d respectively, which has beenprovided with a polyethylene layer 4 on both sides.

The present invention is not limited to the disclosed examples, but maybe modified in many ways that would be apparent to the skilled person,within the scope of the appended claims.

1. A curtain coatable gas barrier coating composition comprising apolymer and a surfactant, wherein the polymer is selected from a groupconsisting of polyvinyl alcohol and polysaccharides, or mixturesthereof, wherein said polysaccharides are soluble or dispersable orsuspendable in water and the surfactant is a water soluble non-ionicethoxylated alcohol.
 2. A coating composition according to claim 1,characterized in that the polysaccharides comprise starch, starchderivatives, MFC (micro fibrillar cellulose) and hemicellulose.
 3. Acoating composition according to claim 2, characterized in that thepolymer comprises polyvinyl alcohol alone or in a mixture with starchand/or a starch derivative and/or MFC and/or hemicellulose.
 4. A coatingcomposition according to claim 2, characterized in that the polymercomprises starch alone, or a starch derivative alone, or a mixturethereof, or either one in mixture with MFC and/or hemicellulose.
 5. Acoating composition according to claim 2, characterized in that thepolymer comprises MFC alone or hemicellulose alone, or a mixturethereof.
 6. The coating composition according to any one of claims 1-5,characterized in that the surfactant is a saturated iso-C13 alcohol. 7.The coating composition according to claim 6, characterized in that thesurfactant has the formula RO(CH₂CH₂O)_(x)H, wherein R═iso-C₁₃H27 and xis 8 or higher
 8. The coating composition according to any one of claims1-5, characterized in that the surfactant is an alkyl polyethyleneglycol ether based on C10-Guerbet alcohol, with a degree of ethoxylationof 7 or higher.
 9. The coating composition according to any one ofclaims 1-5, characterized in that the surfactant is a saturatedshort-chain fatty alcohol.
 10. The coating composition according toclaim 9, characterized in that the surfactant has the formulaRO(CH₂CH₂O)_(x)H, wherein R═a saturated short-chain alcohol and x is 5or higher.
 11. The coating composition according to any one of thepreceding claims, characterized in that it comprises a concentration of0.05-2.,0 weight % of non-ionic ethoxylated alcohol surfactant,preferably 0.1-1.0%, and more preferably 0.2-0.5 weight %, of the totalweight of the bone dry coating composition.
 12. The coating compositionaccording to any one of the preceding claims, characterized in that itcomprises a solid content of the composition of 4-20%; preferably 6-12%and more preferably 8-10%.
 13. The coating composition according to anyone of the preceding claims, characterized in that it comprises platyparticles.
 14. The coating composition according to claim 13,characterized in that it comprises Montmorillonite clay in nano particleform.
 15. The coating composition according to any one of the precedingclaims, characterized in that it comprises a weight percentage ofapproximately 80-99.5% of dry matter in the composition constituted by apolymer chosen as the main component, and that the amount of additivesto the main component, such as other polymers or platy particles andsurfactant, have a weight percentage of approximately 0.05-20% of drymatter in the composition.
 16. A method for providing a substrate with agas barrier layer applied in at least one coating step, comprisingapplication of a coating composition as defined in any one of claims1-15, by curtain coating the substrate with the coating composition inat least one coating step.
 17. The method according to claim 16,comprising curtain coating the substrate in a first step with a coatingcomposition containing polyvinyl alcohol and in a second step with acoating composition containing polyvinyl alcohol.
 18. The methodaccording to claim 16, comprising curtain coating the substrate in afirst step with a coating composition comprising a polysaccharide and ina second step with a coating composition comprising a polysaccharide.19. The method according to claim 16, comprising curtain coating thesubstrate in a first step with a coating composition containing apolysaccharide and in a second step with a coating compositioncomprising polyvinyl alcohol.
 20. The method according to claim 16,comprising curtain coating the substrate in a first step with a coatingcomposition containing polyvinyl alcohol and in a second step with acoating composition comprising a polysaccharide.
 21. The methodaccording to claim 16, comprising curtain coating the substrate in afirst step with a coating composition comprising polyvinyl alcohol, MFC,clay in nano particle form and a water soluble non-ionic ethoxylatedalcohol, and curtain coating the substrate in a second step with acoating composition comprising starch or a starch derivative and a watersoluble non-ionic ethoxylated alcohol or vice versa.
 22. The methodaccording to claim 16, comprising curtain coating the substrate in afirst step with a coating composition comprising polyvinyl alcohol,hemicellulose, clay in nano particle form and a water soluble non-ionicethoxylated alcohol, and curtain coating the substrate in a second stepwith a coating composition comprising starch or a starch derivative anda water soluble non-ionic ethoxylated alcohol or vice versa.
 23. Themethod according to any one of claims 16-22, wherein the coat weight ofthe at least one coating layer is 0.4 g/m² or more and less than 7.0g/m².
 24. The method according to any one of claims 16-23, wherein thecoat weight of the gas barrier layer is 0.8 g/m² or more and less than7.0 g/m².
 25. The method according to any one of claims 16-24, whereinthe substrate is a fibre substrate or a paper or a paperboard.
 26. Acoated substrate having at least one gas barrier layer (2) obtained bycoating a substrate (1) with a coating composition as defined in any oneof claims 1-15.
 27. The coated substrate according to any one of claims26, wherein the gas barrier layer (2) has a coat weight of 0.8 g/m² ormore and less than 7.0 g/m².
 28. The coated substrate according to anyone of claims 26-27, wherein the substrate (1) is a paperboardcomprising at least one fibre based layer coated with the coatingcomposition.
 29. A packaging material comprising a coated paperboardaccording to claim 28, further comprising a layer (4) of low densitypolyethylene on each side of the coated paperboard, and having an OTRvalue of less than 40 ml/m²·24 h·1 atm, and preferably less than 10ml/m²·24 h·1 atm.
 30. A liquid package made from a packaging materialaccording to claim 29.